1885.] NATURAL SCIENCES OF PHILADELPHIA. 339 



in form, however close. It is the word itself which determines 

 its standing, and not its signification or derivation. The question 

 is one of authority, and not of propriety or expediency, and it 

 will be seen that the committee who reported the above- 

 mentioned rule to the British Associaton, took the same view as 

 to its effect that we do (Am. Journ., July, 1869, p. 107). Arthroa- 

 canlha is a different word from Arthracanthus, although of the 

 same et3'mology, and of similar construction, and there are other 

 names of recognized standing in natural history, which bear a 

 closer resemblance to prior names than this. 



Another bibliographic question arises as to the species of this 

 genus. Hall made a good figure of the type, which he called PL 

 punctobrachiatus, but his plates were not published. Williams, 

 however, when establishing the genus gave a brief but very clear 

 definition of the characters of Hall's type specimen (Proc. Am. 

 Phil. Soc, 1883, p. 83), and proposed for it the name Arthroa- 

 cantha punctobrachiata. On p. 86 he again defined its principal 

 characters by comparison with A. ithacensis. The " definition " 

 necessary to impart authority to a published zoological term im- 

 plies a " distinct exposition of essential characters." (See Com- 

 mittee's Report on Rule 12, Am. Journ., 1869, p. 102.) This was 

 given by Williams far better than has been done in a great many 

 specific descriptions of well known Crinoids. It is our opinion, 

 therefore, that A. punctobrachiata is a good species, and that it 

 must be credited to Williams. Whether Hinde's species is iden- 

 tical with A. punctobrachiata we cannot undertake to determine 

 without more direct comparison of specimens. We have exam- 

 ined specimens from the Hamilton group of Ontario, Canada, 

 which undoubtedly belong to A. punctobrachiata, and it is not 

 improbable that A. Carpenter i may prove to be the same thing. 



Arthroacantha is closely allied to Hexacrinus, from which it 

 differs in having three primary radials instead of two, and mova- 

 ble spines along the surface of the plates. That the spines, which 

 are frequentty found in close proximity to the plates, are not 

 mere broken parts of the plates, but constitute independent struc- 

 tures, is clearly seen from Prof. Williams' specimens, which he was 

 good enough to send us for examination. The nature of the 

 spines was disclosed to us more satisfactorily in specimens of A. 

 punctobrachiata from the Hamilton of Canada, in which not only 

 the calyx, but also portions of the arms were preserved, and in 



